The present invention relates to power supply systems and operating methods therefor, and more particularly, to uninterruptible power supply (UPS) Systems, voltage regulators and voltage regulation methods.
Uninterruptible power supply (UPS) systems arc used in a wide variety of applications, such as for providing reliable and regulated AC power to computer systems, telecommunications switching equipment and cable television transmission equipment. Typical UPS systems include batteries, standalone generators or other alternate power sources that may be brought on-line should an AC utility source fail to meet predetermined voltage or other performance criteria, e.g., under xe2x80x9cbrownoutxe2x80x9d and xe2x80x9cblackoutxe2x80x9d conditions. In addition, UPS systems commonly include power conditioning circuits that are designed to reduce spikes, frequency variations, voltage excursions and other irregularities that are often present on AC utility lines. Accordingly, AC voltage regulation is often a key function of a UPS system, an accordingly, most conventional UPS systems include some type of AC voltage regulation circuit.
A UPS voltage regulation circuit is often designed to meet the particular demands of the environment in which the UPS operates. For example, in cable television applications, UPS""s are typically used to supply AC power for distribution over the cable network. As television signals tend to have much higher frequencies than the typical 60 Hz power signals used in a cable system, AC power signals are typically distributed on the coaxial cable itself, multiplexed with television signals, with units such as signal repeaters receiving power from the cable through the used of tuned tank circuits tied to the coaxial conductors. A single pole-mounted cable UPS system may supply a cable network including several such units distributed over multiple branches and large geographical areas.
Because of the widely distributed nature of typical cable television power systems, it is generally inconvenient to de-energize the entire network served by a UPS system when a portion of the network fails. Instead of turning off the UPS system, service personnel typically short out the coaxial conductors leading into a suspected failed portion of the network, replace suspected components, and then remove the short to see if the replacement remedies the failure. Because of this practice, and because of the likelihood of unintentional shorts due to falling tree branches and the like, it is generally desirable that cable television UPS systems exhibit good short circuit current limiting characteristics.
Other environmental factors affect the choice of voltage regulation techniques as well. For example, cable television and telephone systems are very susceptible to lightning and switching induced transients that can cause both short term and long term voltage surges that can damage equipment. Although line equipment typically includes short term voltage spike protection devices such as metal oxide varistors (MOV""s), these devices generally are not very effective at suppressing longer term voltage excursions, e.g., multi-cycle voltage surges such as those caused by load shedding. Accordingly, it is also generally desirable for UPS systems serving such equipment to exhibit excellent surge suppression characteristics.
Several different types of AC voltage regulators have been used to meet UPS operational demands. Many UPS""s employ self-regulating ferroresonant transformer regulators, which have long been used in telephony and similar systems because of their simplicity, ruggedness and desirable operating characteristics. Ferroresonant transformer regulators typically provide excellent voltage regulation, excellent output current limiting under fault conditions, and substantial surge suppression. However, ferroresonant transformer regulators also typically exhibit lower efficiency than comparable linear transformer regulators and tend to be susceptible to subharmonic overshoot and ringing when presented with load step changes and low frequency input discontinuities. Because of the presence of additional windings and the need for a relatively large resonating capacitor, ferroresonant transformer regulators also tend to have greater size and weight than linear transformer regulators of comparable power-handling capability, and thus tend to be more expensive.
Other conventional UPS""s employ voltage regulators that utilize a linear transformer instead of a ferroresonant transformer, relying on additional control electronics to provide the desired regulation. Linear transformer regulators tend to have higher efficiency than comparable ferroresonant transformer regulators, and typically exhibit less overshoot and subharmonic ringing when perturbed by line or load discontinuities, but typically exhibit less than desirable surge suppression or current limiting characteristics.
In light of the foregoing, it is an object of the present invention to provide uninterruptible power supply (UPS) systems that can provide a desirable combination of regulation, efficiency, surge suppression, and short-circuit current limiting.
It is another object of the present invention to provide voltage regulators and methods of voltage regulation that can provide a desirable combination of regulation, efficiency, surge suppression, and short-circuit current limiting.
It is yet another object of the present invention to provide ferroresonant voltage regulators having desirable size and weight characteristics.
These and other objects, features and advantages are provided according to the present invention by voltage regulators, UPS""s and voltage regulation methods in which an input terminal receiving an AC voltage, e.g., a utility AC voltage, is selectively coupled to an input winding of a ferroresonant transformer responsive to at least one of a voltage at the input terminal and a current in the output winding of the ferroresonant transformer. In preferred embodiments, a voltage per turn on the input winding is controlled by selectively connecting the input terminal of the voltage regulator to a location along the input winding, e.g., by tap changing via relays or other switching elements. Additional output control may be achieved by controlling the resonance of the ferroresonant circuit associated with the ferroresonant transformer, for example, by adding or removing, capacitance from the ferroresonant circuit.
The present invention arises from the realization that by regulating the voltage per turn on the input winding of a ferroresonant transformer regulator using, for example, relatively simple tap-changing techniques, the saturation characteristic of the ferroresonant transformer regulator circuit can be controlled to provide improved performance. Accordingly, the ferroresonant transformer regulator circuit may be designed to operate with less circulating current in its resonating circuit while obtaining many of the desirable characteristics of ferroresonant transformers, such as inherent surge suppression and current limiting. Reducing circulating current allows the size of the windings, capacitors and other components to be reduced, and can lead to increased efficiency. UPS operation can be achieved by switching the input winding of the ferroresonant transformer between an AC power source and a battery-powered inverter, or by coupling the AC power source and the inverter to respective first and second input windings of the ferroresonant transformer.
In particular, according to the present invention, an uninterruptible power supply system for producing an AC voltage from at least one of a DC power source or an AC power source includes an input terminal configured to receive an AC voltage from an AC power source, and an inverter operative to produce an AC voltage, preferably regulated, at an output thereof. A ferroresonant transformer circuit includes a transformer having an input winding, an output winding, and a third winding that forms part of a resonant circuit that produces saturation in the output winding when an AC voltage on the input winding exceeds a predetermined amplitude. A transformer input control circuit is coupled to the input terminal and to the inverter output and is operative to couple at least one of the input terminal and the inverter output to the input winding. The transformer input control circuit variably couples the input terminal to the input winding responsive to at least one of a voltage at the input terminal, a current in the output winding, a voltage on the output winding and a current at the input terminal.
According to aspects of the present invention, the transformer input control circuit is operative to increase a voltage per turn on the input winding responsive to at least one of an increase in a current in the output winding, a decrease in a voltage at the input terminal, and a decrease in a voltage on the output winding, and to decrease a voltage per turn on the input winding responsive to at least one of a decrease in a current in the output winding, an increase in a voltage at the input terminal, and an increase in a voltage on the output winding. The transformer input control circuit may be operative to increase a voltage per turn on the input winding responsive to a current in the output winding increasing above a first predetermined threshold and to decrease a voltage per turn on the input winding responsive to a current in the output winding falling below a second predetermined threshold. The transformer input control circuit may also be operative to decrease a voltage per turn on the input winding responsive to a voltage at the input terminal increasing above a first predetermined threshold and to increase a voltage per turn on the input winding responsive to a voltage at the input terminal falling below a second predetermined threshold. The transformer input control circuit may also be operative to decrease a voltage per turn on the input winding responsive to a voltage on the output winding increasing above a first predetermined threshold and to increase a voltage per turn on the input winding responsive to a voltage on the output winding falling below a second predetermined threshold.
In embodiments according to the present invention, the transformer input control circuit is operative to connect the input terminal to the input winding, at a location along the input winding selected responsive to at least one of a voltage at the input terminal, a current in the output winding, a voltage on the output winding and a current at the input terminal. For example, the input winding may have a plurality of taps, with the transformer input control circuit being operative to connect the input terminal to a selected one of the plurality of taps responsive to at least one of a voltage at the input terminal, a current in the output winding, a voltage on the output winding, and a current at the input terminal.
In other embodiments according to the present invention, the system may further comprise a resonance control circuit coupled to the third winding and operative to control a resonance of the resonant circuit to thereby control a saturation characteristic of the output winding. The resonance control circuit may comprise means for increasing and decreasing capacitance in the resonant circuit.
A voltage regulator according to the present invention comprises an input terminal configured to receive an input AC voltage, and a ferroresonant transformer circuit including a transformer having a input winding, a output winding, and a third winding that forms part of a resonant circuit that produces saturation in the output winding when an AC voltage on the input winding exceeds a predetermined amplitude. A transformer input control circuit is coupled to the input terminal and operative to variably couple the input terminal to the input winding responsive to an operating parameter of the voltage regulator, such as a voltage at the input terminal, a current in the output winding, a voltage on the output winding, and a current at the input terminal.
According to method aspects of the present invention, an AC voltage source is variably coupled to the input winding of a ferroresonant transformer circuit responsive to an operating characteristic of the ferroresonant transformer circuit, such as a voltage at the AC voltage source, a current in the output winding of the ferroresonant transformer circuit, a voltage on the output winding, and a current at the AC voltage source. The AC voltage source may be variably coupled to the input winding by connecting an input terminal connected to the AC voltage source to the input winding at a selected location along the input winding, e.g., at a selected one of a plurality of taps. To provide further control, a resonance of the resonant circuit of the ferroresonant transformer circuit may be varied to thereby control a saturation characteristic of the output winding.